The DARPA Packet Radio Network Protocols

Overview/Main Points

Note: I am only describing major differences between this and the
older advances paper - readers should
first read the older version.

Autonomy of Packet Radios (PRs): The PRs are completely
standalone units that communicate with a digital counterpart; the
digital counterpart implements transport and above layers (TCP, etc)
while the PR units implement the network and below layers (IP, etc).
The PR OS/firmware can be loaded over an RS-232 link or over the
radio link; debugging can be done similarly.

Network size: In this paper, it is stated up front that the
PRnet was designed with at most 138 &quotentities&quot in mind - entities
include PRs, repeaters, etc. Larger networks can be created by
piecing PRnets together with gateways.

Route calculation: Each PR maintains 3 tables:

neighbor table - the neighbor table contains one entry per
neighbor PR (a neighbor is exactly 1 hop away). It is built via
the exchange of &quotPROP&quot messages - each PR sends a PROP
message out every 7.5s. Each entry also contains link quality
values (quality changes have hysteresis to avoid yo-yoing).

tier table - each neighbor is said to be at tier 1 from a PR.
All PR's that are exactly 2 hops from a PR (and thus not in tier 1) are at
tier 2 from the PR. The tier table contains a list of all known
destination PRs in the network, along with each destination PR's tier and
the next PR along the best route to that destination PR. Again, PROP
messages are used to build the tier table; PROP packets thus contain
subsets of the PROP originator's tier table. Changes in link connectivity
are reflected in tier tables; care must be taken to not introduce loops.

Packet Headers: An end-to-end (ETE) header contains
source and destination IDs, and a type of service flag. The routing
header (encapsulating the ETE header) contaings source and destination
IDs, sequence numbers, and state modified by intermediate PRs for
routing purposes. This state includes the previous PR router ID
and transmit count, the transmitting PRs ID and transmit count,
and the next PR ID along the route. Also included in this state
are flags used when the best route fails - an &quotalternate routing
request flag&quot indicates that the intended next PR is not acknowledging
receipt, and that any PR of the same or lower tier than the intended
next PR can forward the packet instead. The &quotlateral alternate
routing flag&quot indicates that this has taken place, and that the
next PR to forward the packet must have a strictly lower tier number
to prevent packets perpetually circulating at a constant tier.

Pacing: A packet send from PR L to PR M experiences a triple
delay; the time to go from L to M, the time to go from M to the next PR N
(which includes an implicit acknowledgment), and the time for the implicit
acknowledgement from N back to M. This third delay is necessary because M
can only receive from one radio at a time. Successive packets from a given
PR are thus paced with this triple forwarding delay imposed on them.
Also, a second packet to the same next-hop PR is not sent until the
first packet is acknowledged; this is done for congestion and flow
control, and is called single-threading.

Fairness queues: The normal FIFO packet ordering from a
PR is altered to expedite packets from a PR with poor radio connectivity
to maintain fairness.

CSMA: CSMA is used, with random delays imposed once a channel
becomes free and (exponential?) backoffs in case of collisions.

Relevance

This paper is not as ground-breaking as its predecessor, but some reasonably
interesting improvements and modifications to the original PRnet
are described.

Flaws

Maintaining complete routing tables for the entire network at each
node seems wasteful; this must be why they claim a maximum of 138
network entities.